The covalent attachment of polyethylene glycol (PEG) to proteins in sufficient amount renders them nonimmunogenic by various criteria. We propose to modify the following enzymes of potential use in cancer therapy and to test their in vitro and in vivo activities against tumor cells: various asparaginases, phenylalanine ammonia-lyase, arginase, and the xanthine oxidase, uricase and adenosine deaminase triplet. PEG-adenosine deaminase may also find use in treatment of combined immunodeficiency disease. Alpha- and beta-glucosidases, alpha- and beta-galactosidases, and N-acetyl-beta-D-glucosaminase are to be modified and tested for efficacy in treating storage diseases. We will modify heterologous immunoglobulins so as to eliminate immunogenicity while retaining their antibody function. Synthetic allergens will be modified to produce a partially antigenic PEG-allergen that preferentialy produces blocking antibodies.